Dry eyes (keratoconjunctivitis sicca, KCS) is a disease characterised in reduced secretion of tears. Cornea and conjunctiva suffer from an inferior function of tear supply resulting in structural changes as epithelial cell death, corneal infiltrations and others. Patients are afflicted with smarting pain, sensations of dust, occasionally reduced vision, aches, irritation and so on. In its classical form KCS appears as Sjogren's syndrom involving engagement of other serous glands but KCS is much more common in its less serious form. It has been estimated that about 25% of the patients in an eye clinic suffer from more or less chronically dry eyes (Scharf, J., Zonis, S., Perelman J. and Hit, E., Harefuah., 1975, 89, p 505).
Treatment of dry eyes is today a major problem in ophtalmology. So far tear substitutions have mostly been offered to patients. Tear substitutions normally consist of salt solutions with a lubricant e.g. methyl cellulose, polyvinyl alcohol and hyaluronic acid. One important disadvantage of this treatment is its very short time of action (Duke Elder, Systems of Ophtalmology Volume XIII Part II p 634, 1974). Another disadvantage of most tear substitutes is that they contain benzalkonium, which has a damaging impact on eyes of mammals (Pjister, R. R. and Burstein, N., Invest Ophtalmol. 15:246, 1976). Also, many pharmaceutical formulations are very poorly adapted to the natural specific composition or electrolytic status of natural tear fluid (Gilbard, J. P., Rossi, S. R. and Heyda, K. G., American J of ophtalmology 10 107:348-355, 1989).
Carbachol chloride is a well-known substance in the treatment of glaucoma. For this indication its ability of lowering intraocular pressure is employed. Here, carbachol chloride is normally used in a concentration of 3% and in combination with benzalconium chloride in order to obtain good penetration of the eye (Smolen, V. F., Clevenger, J. M., Williams, E. J. and Bergdolt, M. W., J of Pharmaceutical Sciences, 1973, 62 p 958). Other than having this effect, benzalconium chloride in pharmaceutical mixtures has a preservative function. Other effects of carbachol chloride exposure to the eye are tear secretion, miosis and influence on the accomodatory ability of the eye. These so called cholinergic effects are in this context considered as side effects.
Substances having the same effects as carbachol chloride are methacholine chloride, methanecholine bromide, bethanechol chloride, furtrethonium iodide and arecholine.
Tear production can be stimulated also by other cholinergic substances such as acetylcholine, pilocarpine, physostigmine, neostigmine and others. These compounds, however, easily penetrate the cornea upon exposure, thereby activating cholinergically innervated intraocular structures such as the pupillary sphincter and the accomodation muscle, both unwanted side effects in cases where only extraocular effects are wanted. Such compounds, therefore, are less suited for this purpose than is carbachol.